1. Field of the Invention
The present invention generally relates to devices used to electronically mark and locate buried objects, and more particularly to a nonconductive tape or cord for dispensing a plurality of passive electronic markers at predetermined intervals adjacent to an underground utility.
2. Description of the Prior Art
Buried conduits are employed for supplying a wide variety of utilities, including pipelines for gas, water and sewage, and cables for telephone, power and television. It often becomes necessary to locate defective or damaged cables, pipes, etc., in order to repair or replace them. Conversely, it is important to know with as much accuracy as possible the approximate vicinity of such items in order to avoid disturbing them when digging or excavating for other purposes. Aboveground marking devices may be installed immediately after the conduit is buried, but they are often lost, stolen, or destroyed after a short period of use.
In the past, three different concepts have been used to indicate the presence of buried conduits, namely, warning tapes, trace wires, and electronic marker systems. A warning tape is simply a band of plastic which is placed above the conduit before burial. These tapes are used to alert the excavation team of the presence of the conduit before any damage thereto might occur. As the backhoe or other mechanical digger excavates the site, it will hopefully uproot a portion of the warning tape prior to contact with the conduit. The primary disadvantage of warning tapes is that they cannot be detected by any surface instrumentation.
A single trace wire is sometimes buried with a utility line. The trace wire is used as a conductor for an AC signal which radiates an electromagnetic field above ground. The electromagnetic field may be detected with an appropriate receiver, and the underground path of the line thereby determined. Although the conduit itself may act as a conductor (i.e., when steel pipe is used), most conduits are non-conductive and therefore require a trace wire. There are three significant disadvantages in the use of a trace wire. First of all, it is necessary to provide above ground access to the trace wire in order to couple the AC signal thereto. Secondly, if a break occurs in the wire (due to excavation, or natural causes such as corrosion, earth movement or burrowing mammals), then the wire becomes useless. Finally, the trace wire is too thin to imprint a warning message thereon, precluding any visual warning. Additionally, a receiver cannot distinguish the trace wire from any other conductor in the vicinity.
Electronic marker systems for locating buried objects are known in the art, and generally consists of two types, namely, active and passive markers. Active markers require the use of a power supply which amplifies a signal source (usually an AC signal). The signal is radiated by the underground marker and detected by a receiver unit above ground. Passive markers, in contrast, have no power supply, but rather operate in a resonant mode, responsive to a transmitted electromagnetic field.
A passive marker is basically a wire coil and capacitor surrounded in a protective envelope, which is then buried adjacent to the cable, pipe, or other object to be located. The marker is self-contained, with no external, accessible connections. Minnesota Mining & Manufacturing Co. (assignee of the present invention, and referred to hereinafter as "3M") markets several kinds of passive markers for different applications as part of its ScotchMark Electronic Marker System ("ScotchMark" is a registered trademark of 3M). See also U.S. Pat. No. 4,761,656 issued to Cosman et al. on Aug. 2, 1988.
Passive markers are activated by radiating a signal into the ground in the area where the marker is expected to be found. The signal is emitted via an inductive coil held close to the surface. When the coil is directly over, or near, the passive marker (which is itself an inductive coil), the marker accepts energy within its bandpass and stores it, reaching a sustained amplitude during the transmission cycle. When the transmission cycle ends, the marker re-emits the signal with an exponentially decaying amplitude. A second coil within the transceiver unit acts as a receiving antenna which detects the re-radiated signal, alerting the locating technician with an audible tone or other indicator means.
Warning tapes, as well as passive electronic markers, are usually color-coded according to the particular type of utility line they mark. Specifically, gas line markers are yellow; telephone cable markers are orange; waste water tunnel markers are green; water pipe markers are blue; and power supply markers are red. Similarly, the passive marker is "coded" by tuning the coil for a specific resonant frequency. Five distinct frequencies have been designated: 83.0 kHz for gas; 101.4 kHz for telephone; 121.6 kHz for sewage; 145.7 kHz for water; and 169.8 kHz for power. In this manner, a locating technician searching for, say, a gas line, cannot accidentally activate a telephone marker since his transmitter will only be sending out an 83 kHz signal, which is not within the bandwidth for a telephone marker tuned for 101.4 kHz. Of course, these frequencies have been designated by convention, and are not meant to be restrictive.
One device which incorporates all three of the above concepts is disclosed in U.S. Pat. No. 4,767,237 issued to Cosman et al. on Aug. 30, 1988, and probably represents the closest prior art to the subject invention. One embodiment described in that patent consists of a warning tape which has integral trace wires. The specification also discusses attachment of passive markers on the tape at specific locations, e.g., at a T-joint or cable termination. This device still suffers from the same crucial drawbacks as trace wires, viz., the trace wire/tape must have an access point above ground and, if the wire/tape is severed, it becomes entirely useless. It would, therefore, be desirable and advantageous to devise a marker tape system that is useable without such an access point, and which still functions in spite of any break in the tape.
Accordingly, the primary object of the present invention is to provide a device for electronically marking buried conduits.
Another object of the invention is to provide such a device which does not require an above ground access point.
Still another object of the invention is to provide a marker system which may be buried proximate the conduit along its entire length, but which will remain operative even if severed.
Yet another object of the invention is to provide a marker system which identifies the buried conduit by means of successive passive markers which together form a unique code for that conduit.